That depends on the 427.. and the compression. *factory* 427's ('66-'69) were built with higher compression (10.25:1 or higher as I recall) and would not run very well on 87 octane; probably would not run well even with 93 octane. But your engine has probably been rebuilt by now, so you should check with the seller as to the compression ratio and what fuel he runs. Go for a ride, listen for pinging or detonation... which is bad...

One rule of thumb is the lowest octane that doesn't cause detonation. There are many variables that could matter. Cam and compression for starters. If nothing else try small quantities from high octane to low. When you get detonation, back up to the next higher octane.

You might also stay away from ethanol, too. Stuff loves to eat rubber, nylon, gaskets and gas lines, needle tips, your pets, and has an affinity for water. Find yourself a station in the area that sells real gasoline. Buy a good performance additive, or tetraethyl lead from that guy in Georgia (if he still advertises it). Hard to find hi-performance high octane unleaded fuel at the pump any more - there used to be a couple of stations locally that sold Turbo Blue, but they are long since gone. I miss the smell of Holley aspirated 102 octane leaded gas - my eyes still water from the odor. Probably lost a few brain cells because of it, or maybe that was the other alcohol (non-ethanol).

So did this the same car that did not have a trim plate? Gasoline wise I would agree with has been previously posted, I would say the chances are very slim 87 would be satisfactory for your motor. Even my 396 325hp block which is 10.25 compression ratio (mild) and if I run less than 91 octane it pings and there is obvious pre-detonation while cruising. If you were only going to put gas in it to get it on the trailer or moving it around the shop then 87 may be ok IMO.